>>Dr. Ketchum: So now we are ready to move into the chemical classification of our chemical
messengers. So this is part three of the chemical messenger lecture on intercellular communication.
So in this section, we’re going to look at the overview of the five major classes of
chemical messengers. So again we come back to our outline, and on our outline now we’re
going to focus specifically these classes of chemical messengers. And so this will just
be some general information that you’ll need to commit to memory so that you can apply
this information in subsequent sections. So this table illustrates the chemical classification
of your messengers. And so when you look, here are your five classes of chemical messengers.
This table gives you your chemical property. And so if you know if a chemical messenger
is lipophobic, lipophilic, that’s going to help you determine how that ligand is synthesized
and secreted, how the ligand is transported to the target cell, and how the target cell
will respond. This table is—is a great table, but there could be a lot more information
that could be added to it. So as we go throughout the next series of lectures, I would suggest
thinking about adding additional columns to this table. So for example,
as we go through the classes, we’re going to go through some
general properties. So perhaps you want to add a column here that says “General Properties”
and the information that we discuss you may want to add to this table as we go along.
Let’s just begin that if your solute or your chemical messenger, in other words, is
lipophobic, it must be hydrophilic. And if your solute is lipophilic, it must be hydrophobic.
That’s going to be fundamental for you to remember.
So we begin with the amino acids. We have only four amino acids that are going to be
functioning as messengers; all of these will function as neurotransmitters. So if you
recall, neurotransmitters are a type of functional classification. And so if you think back to
what a neurotransmitter is, it’s a ligand that gets synthesized and secreted from a
presynaptic neuron, and then travels through the interstitial fluid to bind to receptors
on the postsynaptic cell. And so the four amino acids that you should remember that
can function as neurotransmitters are glutamate, aspartate, glycine, and GABA.
Remember that we have 20 different amino acids, however only four are going to be functioning as neurotransmitters.
Then we move into the amine chemical class. And the amines are all going to be derived
from an amino acid. And the reason they’re called amines or they’re put into this class
called amines is that they all contain the amine group NH2. All right, so here are some
examples of amine messengers—and we’re going to be spending quite a bit of time this,
this semester discussing various amine messengers. First off, we have the catecholamines.
So that’s a group of amines that includes dopamine, norepinephrine, which you may have
heard in the past called noradrenaline. The last type of catecholamine is called epinephrine.
Epinephrine, you may be familiar with it, is called adrenaline. We will be using the
term epinephrine in this class.
Now thyroid hormones are also types of amine messengers. However, they are made from two
tyrosine amino acids. If you look at the catecholamines, they are only derived from one tyrosine, not
two. Thyroid hormones are derived from two tyrosine amino acids. Histomine, which you
saw as an example of a paracrine chemical messenger, histamine is a type of amine messenger,
and it’s derived from histadine. And then there’s serotonin, which is made from the
amino acid tryptophan. And so what you need to do is start organizing this materials in
a way such that when we move into thyroid hormone, for example, when I say thyroid hormone
you can say, “Okay, I know that that’s an amine and that’s a chemical class of a chemical
messenger.” We’ll be using a lot of these amine messengers throughout this semester.
Then we move onto the peptide and the protein chemical class. So you’re already familiar
with what a protein is, and remember, a protein is a chain and amino acids are the subunit
for a protein are amino acids. This is going to be our most abundant type of ligand. So
once again when I mention a peptide or a protein to you, my hope is that you will think about
how peptides and how proteins are synthesized via transcription and translation and how
they can go from a pre-pro-peptide to a pro-peptide, to a peptide in the various organelles that
we’ve already discussed. And then we have the steroids. This is the other chemical class
of chemical messengers—steroid ligands. It’s important for you to know that they
are all derived from cholesterol, and it’s very important for you to realize that all
of these are going to be functioning as a hormone. So will steroid hormones travel through
the blood? Hopefully you answered yes; all hormones travel through the blood.
Our next chemical class are the eicosanoids. The eicosanoid ligands are all derived from
arachidonic acid. So this is actually within the cell membrane. All of these chemical messengers
are involved in paracrine signaling. Do paracrine signals or paracrine messengers
travel through the blood? The answer is no. None of your paracrine messengers will travel
through the blood. Here’s some examples of eicosanoids, and they all function as paracrine
signallers. So prostaglandins and leukotrienes—these are very important in inflammation. Thromboxanes
are another example, they are important in blood clotting.